1. Field of the Invention
The present invention relates generally to improving the toughness of low-carbon, high-strength steels and, more particularly, to a method of post-solidification processing to minimize the content of coarse grain-refining precipitates that may form during solidification in low-alloy and alloy high-strength steels containing approximately 0.09-0.17% by weight C.
2. Description of the Related Art
The toughness of grain-refined, high-strength steels is highly dependent on the content of coarse AlN precipitates in the microstructure, as discussed in M. J. Leap et al., SAE Paper 961749, 1996; and M. J. Leap et al., 38th Mechanical Working & Steel Processing Conference Proceedings, Iron and Steel Society, Inc., 1996, pp. 195-220. It is also known that coarse AlN precipitates degrade the toughness of high-strength steels over a broad range of test temperature by providing preferential sites in the microstructure for the formation of transgranular cleavage facets, quasi-cleavage facets, and secondary microvoids. This behavior is disclosed in several publications: M. J. Leap et al., ASTM STP 1259, 1997, pp. 160-195; M. J. Leap et al., 39th Mechanical Working & Steel Processing Conference Proceedings, Iron and Steel Society, Inc., 1997, pp. 685-703; and M. J. Leap et al., Metallurgical Transactions A, vol. 30A, 1999, pp. 93-114. Although high-strength steels containing less than about 0.1 wt. % C or between 0.2 wt. % C and 0.4 wt. % C may exhibit AlN precipitates up to about 400 nm in size, it is known that these precipitates can be dissolved during the reheating operation that precedes finish rolling or forging. U.S. Pat. No. 5,409,554, for example, describes a process for optimizing toughness in which reheating for the last hot-working operation is conducted at a temperature in the vicinity of the least soluble species of grain-refining precipitate in a steel.
It has been more recently found that grain-refined, high-strength steels containing approximately 0.09-0.17 wt. % C may exhibit extremely coarse AlN precipitates after hot working and heat treating. The coarse precipitates, which degrade the toughness of tempered martensite, are not readily eliminated from the microstructure with thermal/thermomechanical processes of the prior art. However, many low-alloy and alloy steels containing about 0.09-0.17 wt. % C are utilized in structural, mining, and oil field applications that have demanding toughness requirements, and the development of marginal toughness in combination with high variability in toughness has exemplified the need to improve the toughness of these steels in various products.
An example of the toughness degradation that results from the presence of extremely coarse grain-refining precipitates in tempered martensite is provided by two grain-refined, high-strength 9313M steels, the compositions of which are listed in Table 1. Sections of wrought bar from the two heats, designated steels A and B, were forged, carburized at 954° C. for eight hours, reaustenitized at 829° C. for two hours, oil quenched, and tempered at 204° C. for four hours. Charpy V-notch specimens were extracted from the forged and heat treated components in the longitudinal orientation and tested at room temperature in accordance with ASTM E23. Steels A and B, while possessing similar compositions, microstructure, and strength, exhibit room-temperature impact toughness values of 61 J and 14 J, respectively. The large difference in the toughness of the two steels primarily reflects different amounts of ductile crack extension from the notch root of Charpy specimens prior to unstable fracture. The abrupt change in fracture mode to cleavage/quasi-cleavage (steel B) and cleavage/quasi-cleavage intermixed with small amounts of ductile rupture (steel A) results from the presence of particles larger than a critical size that is defined in terms of the strength and strain hardening capacity of the matrix microstructure. Low densities of small to intermediate-sized TiN precipitates are observed on the fracture surfaces, particularly in regions of ductile rupture on the steel A specimen. However, both steels exhibit a preponderance of coarse AlN precipitates on the fracture surfaces of the Charpy V-notch specimens, which indicates that coarse AlN precipitates are the microstructural feature responsible for the initiation of a majority of local fracture events that produce cleavage/quasi-cleavage facets, FIG. 1.
TABLE 1Steel Chemistries (weight percentages) N  OSteelCMnSiCrNiMoSPTiAl(ppm)(ppm)A0.140.690.221.453.230.10 0.006 0.0090.002 0.023  92  8B0.150.700.241.433.250.120.0020.0090.0030.026966
Size distributions of AlN precipitates located in the unstable fracture region of Charpy V-notch specimens are shown in FIGS. 2a and 2b for the two steels. The dispersion of AlN precipitates on the fracture surface of the Charpy specimen with comparatively high toughness (steel A) exhibits a mean size of 132 nm, whereas the steel B specimen exhibits a somewhat coarser AlN dispersion with a mean size of 169 nm. A majority of the AlN precipitates in both steels are less than 400 nm in size, although low densities of AlN precipitates as large as 1 μm are present on the fracture surface of the steel B specimen. The steel B specimen also exhibits a higher area density of precipitates on the fracture surface than the steel A specimen.
Second-phase dispersions in metals typically exhibit a log-normal distribution of feature size when the formation of the dispersion is governed by a single mechanism over a range of temperature. However, the grouped size data for the two steels exhibit significant departures from log-normal behavior at large precipitate sizes, FIG. 2c. The bilinear nature of the cumulative size distributions indicates that the extremely coarse AlN precipitates form at drastically different temperatures than the smaller precipitates in both steels. This observation is corroborated by thermodynamic calculations that predict the presence of TiN and AlN in the solute-enriched interdendritic liquid during solidification in a 9313M base composition containing aluminum and nitrogen in contents representative of grain-refining additions, FIG. 3. Since solute enrichment is driven by large differences in the solubility of alloying elements in the solid and liquid phases, the general precipitation reaction is prevalent in steels containing substantial amounts of δ ferrite during solidification, i.e., steels containing about 0.09-0.17 wt. % C.
The data for steels A and B of Table 1 indicates that increases in the size and content of the coarsest AlN precipitates in a dispersion degrade toughness. Thus, the precipitation of AlN during solidification is particularly important in that substantial contents of extremely coarse precipitates may form in air-melt steels with aluminum in concentrations representative of a grain-refining addition, i.e., 0.005-0.050 wt. %. The present invention addresses the need to improve the toughness of air-melt, high-strength steels containing about 0.09-0.17 wt. % C by providing a commercially viable process for minimizing the content of extremely coarse AlN precipitates in the tempered martensitic microstructure of the final product.